This is a very useful albeit a bit geeky app for Android phones, SPB Software released SPB Wireless Monitor that can measure data traffic via all types of connections and calculating network usage costs according to the current data plan. Here is more info on this new Android app:

SPB Wireless Monitor for Android Main Features:

- Per application traffic reports or time-based reports - Daily, monthly, yearly or custom period cost reports - The reports can be viewed on the screen, shared or exported as CSV files - CDMA, GPRS, 3G, and Wi-Fi connection supported - Peak and off-peak tariffs support with 300+ different service plans from various countries - Data usage alarm - Home screen widget